Currently, attempts are being made to promote the research and development and practical application of clean energy from the standpoint of protecting the environment, and solar cells have attracted attention since solar light, which is an energy source, is inexhaustible, pollution free, and more. In the past, bulk solar cells of single crystal silicon or polycrystalline silicon have been used as solar cells; however, bulk solar cells need a large amount of energy and time for the growth of crystals, and complicated processes are required in the subsequent manufacturing processes such that the manufacturing costs increase and the productivity is low, and therefore there is an urgent demand for the development of solar cells which use as little silicon as possible.
Hence, active efforts are being made to develop a thin film solar cell for which a semiconductor of amorphous silicon or the like having a thickness of, for example, 0.3 μm to 2 μm is used. Since a thin film solar cell has a structure in which a necessary amount of a semiconductor layer for photoelectric conversion is formed on a glass substrate or a heat-resistant plastic substrate, the thin film solar cell is advantageous in terms of being thin, lightweight, and low cost, having an area which is easily increased, and the like.
For the thin film solar cell, there are a super-straight structure and a sub-straight structure, and the super-straight structure generally employs a multilayer structure, in which a substrate, a transparent electrode, a photoelectric conversion layer, and a rear surface electrode are sequentially formed, since solar light is made to be incident from the translucent substrate side. Here, in a case in which the photoelectric conversion layer is constituted by an Si-based material such as an amorphous Si film or a thin film polycrystalline Si film, since the extinction coefficient of the photoelectric conversion layer is relatively small, in a commonly used film thickness of several micrometers, some of the incident light permeates the photoelectric conversion layer, and does not contribute to power generation. Therefore, in general, a conductive reflective film is used for the rear surface electrode so as to reflect light which is not absorbed and return the light to the photoelectric conversion layer, thereby improving the power generation efficiency.
For the thin film solar cell, in the past, the transparent electrode and/or the conductive reflective film have been formed using a vacuum film forming method such as sputtering; however, in general, huge costs are incurred in the introduction, maintenance, and operation of a large-scale vacuum film forming apparatus. In order to solve the above problem, there is a technique in which non-electrolytic plating is carried out to produce a conductive reflective film which is formed using a wet film forming method for use in solar cells (Japanese Patent Application Laid-Open (kokai) No. H05-95127).
However, the non-electrolytic plating method employs processes, in which a plate protection film is formed on the surface side, a pretreatment is carried out using an HF solution on a side on which a plating treatment is to be carried out, the subject is immersed in a non-electrolytic plating fluid, and the like, the processes of the method are problematic, and it becomes necessary to treat the liquid waste.
Next, as a method that is more convenient than the non-electrolytic plating method, research is being carried out into a method in which a metal having high reflectivity such as silver is made into nanoparticles, and the particles are coated onto the surface (Japanese Patent Application Laid-Open (kokai) No. H09-246577); however, generally, there is a tendency for the reflectivity from the rear surface side to decrease compared to the reflectivity from the front side.
Research is also being carried out into a conductive reflective film which overcomes the above disadvantages, and is formed on a base material by firing metal nanoparticles, in which the average diameter of pores appearing in the contact surface of the film on the base material side is 100 nm or less, the average depth at which the pores are located is 100 nm or less, and the number density of the pores is 30 particles/μm2 or less (Japanese Patent Application Laid-Open (kokai) No. 2008-288568).
However, it has been found that a conductive reflective film in which the average diameter and the like of the pores appearing in the contact surface on the base material side are controlled has high reflectivity from the rear surface side, but film stress is caused during the firing of the nanoparticles, and therefore there is little possibility to improve the adhesion properties of a stand-alone metal nanoparticle-sintered film with respect to the base material.